The present invention relates to coded modulation techniques and, more particularly, the use of such techniques in fading channel applications, e.g., digital cellular mobile radio.
During the past decade, trellis-coded modulation has proven to be a practical power-efficient and bandwidth-efficient modulation technique for channels with additive white Gaussian noise (AWGN). This technique has now been widely used in commercial telephone-line modems and has resulted in an increase of line rates of those modems to as much as 19.2 Kbits/s.
More recently, those in the art have investigated the applicability of trellis-coded modulation to a further class of channels--specifically, fast-fading channels, i.e., channels in which the signal amplitude can vary so drastically over short time intervals that it is not practical to track it and thereby to accurately recover the transmitted information. Indeed, the signal amplitude may be so weak that, even if it could be tracked, accurate data recovery may, again, not be possible. Mobile radio channels of various types fall within this category. As in telephone-line modem applications, the use of trellis codes in such channels provides so-called "coding gain" in signal power (compared to so-called "uncoded" modulation approaches). The ultimate result is an enhanced capability for accurate information recovery without requiring additional signal bandwidth. Unfortunately, it turns out that the improvement in error rate performance achieved for a given amount of coding gain is significantly less for the fast-fading channel than for, say, the telephone-line channel. For example, 3 dB of coding gain can provide as much as three orders of magnitude improvement in the error rate for the telephone-line channel, but only about a factor-of-three improvement for the fast-fading channel. This disparity arises principally out of the very nature of the fast-fading channel, i.e., its fading characteristics.